The present invention relates to spacers. More specifically, the present invention relates to spacers acting between a soil grade and the bottom of a form for a concrete foundation member, such as a slab or beam, the spacers being collapsible when the soil under them swells due to water resorption and the like.
Construction of the foundation of a building generally includes the steps of: excavating a foundation pit; placing pilings; digging trenches between the pilings and pouring concrete beams in the trenches; and pouring a reinforced concrete foundation slab over the beams and onto the soil grade between the beams.
Problems exist with the above mentioned construction method in that in certain soil conditions, for example in dense clay soils, the soil in the excavated pit will dry out, thus shrinking, during the time span between excavation and the pouring of the foundation members. Eventually, once the foundation members are poured and set, the soil will resorb water and re-expand. This re-expansion of the soil generates significant forces on the foundation members which it contacts. In many circumstances these forces are sufficient to heave, crack or shatter the slab and/or beams of the foundation.
Previous attempts have been made to solve this problem by providing a spacer between the concrete foundation members and the soil. One prior art technique to provide this spacer employs a layer of corrugated cardboard boxes which are placed on the soil. The upper surface of the boxes function as the lower surface of the form for the foundation member and the concrete is poured onto them. When the soil is subsequently infused with water, the soil expands into the void between the member and the soil created by the boxes, crushing the boxes, but avoiding cracking or breakage of the slab or beam. A box for use in this technique is shown in U.S. Pat. No. 4,685,267 to Workman.
However, problems exist with this technique in that it is labour intensive to fold and place the boxes on the soil. It is also difficult to prevent the boxes from becoming damp and collapsing prior to pouring or setting of the concrete members.
Attempts have been made to overcome these difficulties by employing resilient polystyrene foam slabs instead of corrugated cardboard boxes. However problems exist with this technique as well in that the polystyrene foam slabs are relatively expensive and they are resilient when compressed. Specifically, the polystyrene foam slab will be compressed between the soil and the underside of the foundation member as the soil expands. Due to its resilient nature, as the slab is compressed it generates a reaction force between the bodies compressing it and once the soil has expanded to the point where the reaction force produced by the slab is sufficient, the foundation member will break or heave.
The reaction force produced depends upon the density, uncompressed thickness and amount of compression of the slab, with the force increasing with the amount of compression. It is therefore necessary to increase the uncompressed thickness of the slab to reduce the reaction force produced by a given amount of soil expansion.
For example, a six inch thick slab of polystyrene foam may, depending upon the density of the foam used, be compressible to four inches before a reaction force is produced which would damage a foundation member, thus safely allowing up to two inches of soil expansion to occur. To accommodate three inches of soil expansion, a ten inch thick slab of polystyrene foam may be required, the slab being compressible to seven inches before a reaction force is produced which would damage the foundation member.
As is apparent, any increase in the required safe range of soil expansion will lead to an increase in the volume, and therefore the expense, of the required polystyrene foam slabs. Furthermore, greater excavation of the construction site may be required to accommodate the thicker polystyrene foam members.
It is an object of the present invention to provide a novel spacer which obviates or mitigates at least one of the above-mentioned disadvantages.
According to the present invention there is provided a collapsible spacer of water resistant material for disposition between a soil grade and a form, the spacer comprising voids to allow permanent deformation of the spacer when a predetermined load upon the spacer is exceeded.